1. Field of the Invention
The present invention relates to an exposure apparatus of a projection type or the like for a semiconductor circuit apparatus such as an IC, LSI, and VLSI, and a device manufacturing method capable of using this apparatus. More particularly, the present invention relates to an exposure apparatus of a projection type having an auto-focusing control function a time of repetitively reducing and projection-exposing a circuit pattern of a reticle onto a semiconductor wafer surface, a so-called auto-focusing function and a device manufacturing method using the apparatus, preferably in the field of manufacturing a semiconductor device.
2. Description of the Related Art
In a projection exposure apparatus employed in the manufacture of a semiconductor device, the alignment of a reticle and a wafer at a high precision is required so as to transfer a circuit pattern as a mask, formed on the reticle, onto a photoresist layer on a wafer, glass plate, etc., as a photo-substrate at a high precision of alignment. There has been known a technique such that in a projection exposure apparatus such as a stepper, the alignment of a reticle and a wafer is executed by both reticle alignment to align a reticle with a projection exposure optical system and wafer alignment to align a wafer with the projection exposure optical system. As for a reticle alignment method, there are methods, e.g., an FRA (Fine Reticle Alignment) method, which have been disclosed by the present applicant. As for a wafer alignment method, there has been known a TTL (Through-The-Lens) method of directly aligning the wafer with the projection optical system, an off-axis method of aligning the wafer with an observation optical system having another optical axis different from that of the projection optical system, and the like.
FIG. 1 is a schematic diagram of a projection exposure apparatus having an auto-focusing function by the TTL method, which has been disclosed in Japanese Patent Application Laid-Open No. 9-260269. Referring to FIG. 1, reference numeral 6 denotes a reticle which is held to a reticle stage 7. A projection lens (exposure lens) 9 allows a circuit pattern on the reticle 6 to be reduced to ⅕ or {fraction (1/10)} in size onto a wafer 12 on an xyz stage (a wafer stage) 11, thereby forming an image and exposing it. In FIG. 1, a reference flat mirror 13, in which a surface of the mirror 13 is almost aligned with an upper surface of the wafer 12, is arranged at a position adjacent to the wafer 12. A stage reference mark 13a (mark for detecting focus) in FIG. 2 is formed onto the reference flat mirror 13. The xyz stage 11 can move in an optical axis (z) direction of the projection lens 9 and in a surface (x, y) perpendicular thereto and, of course, be rotated around the optical axis. An illumination optical system shown by component elements 1 to 5 in FIG. 1 illuminates an area in a picture plane of the reticle 6 where a circuit pattern is transferred.
A light-emitting portion of a light source 1 for exposure is positioned at a first focus of an elliptical mirror 2. Light emitted from the light source 1 is made incident to an optical integrator (fly eye lens) 3 where the light incident surface is positioned to a second focus position of the elliptical mirror 2 and a light emitting surface of the optical integrator 3 forms a secondary light source. The light emitted from the optical integrator 3 as a secondary light source illuminates the reticle 6 via a lens for illumination 4 and a field lens 5.
Component elements 10, 13, and 14 form an auto-focusing optical system of an off-axis type. Reference numeral 10 denotes a projection optical system (auto-focusing incident system). Light beams as non-exposing light emitted from the projection optical system 10 are converged and reflected to a point on the reference flat mirror 13 (or an upper surface of the wafer 12). The light beams reflected by the reference flat mirror 13 are made incident to a detection optical system (auto-focusing light-receiving system) 14. A light receiving device for position detection (not shown) is arranged in the detection optical system 14, and constructed so that an incident light point of the light receiving device for position detection is made conjugate to a reflecting point of light beams on the reference flat mirror 13. A positional offset in the optical axis direction of the reduction projection lens 9 of the reference flat mirror 13 is measured as a position offset of incident light beams on the light receiving device for position detection in the detection optical system 14.
Transmitted to an auto-focusing control system 32 is the positional offset from a predetermined reference surface of the reference flat mirror 13, which the detection optical system 14 has measured. The auto-focusing control system 32 instructs a driving system 33 for driving the xyz stage 11, to which the reference flat mirror 13 is fixed, to move toward the z-direction. When a detection optical system 27, as will be explained hereinafter, detects a TTL defocus position, the auto-focusing control system 32 drives the reference flat mirror 13 to move up and down in the optical axis direction (z direction) of the projection lens 9 near a predetermined reference position. The auto-focusing control system 32 also controls a position of the wafer 12 during exposure.
Next, the description turns to component elements for detecting a focusing state on the surface of the wafer 12, driving the wafer stage 11 on the basis of the detected signal, and detecting an in-focus position of the projection lens 9. Reference numeral 27 denotes a TTLAF (Through The Lens Auto-Focusing) detection optical system having component elements 23, 24, 26, 40, and 41, which will be mentioned hereinafter. Illuminating light beams emitted from a fiber 40 pass through a half mirror 41 and are converged near the reticle 6 via an objective lens 24 and a mirror 23. A translucent portion (opening window portion) (not shown) having a predetermined size is set at a position in an area other than an actual device area on the reticle 6. The illuminating light beams pass through the opening window portion and, thereafter, are converged on the reference flat mirror 13 via the projection lens 9. As discussed above, a stage reference mark (mark for focus detection) 13a is marked onto the reference flat mirror 13, as shown in FIG. 2. Reflecting light from the reference flat mirror 13 is returned through an original light path, reflected to the half mirror 41 through the projection lens 9, opening window portion, mirror 23, and objective lens 24 according to this order and is made incident onto a position sensor 26.
The reference flat mirror 13 is arranged onto the wafer stage 11 similar to the wafer 12, and is fixed to the focusing surface, almost matching the wafer 12. The auto-focusing control system 32 controls focus positions of the wafer surface 12a and the stage reference surface 13a of the flat mirror 13 or focus offset amounts between both of these surfaces. As a result, in accordance with a successive sequence, the reference flat mirror 13 is focused and only the supply of a predetermined offset amount results in automatically focusing the actual wafer thereon.
As shown in FIG. 2, the stage reference mark 13a is composed of lines and spaces in the vertical direction, which have a predetermined line width. Light beams emitted from the stage reference mark 13a on the reference flat mirror 13 return in the passed path (in the pass-back path) and reach the objective lens 24. The light beams passing through the objective lens 24 are reflected to the half mirror 41 at a next time, and an image is formed onto a sensor surface 26a of the position sensor 26. The position sensor 26 may be a one-dimensional array sensor or two-dimensional array sensor, e.g., a CCD is a typical sensor. Corresponding to the stage reference mark 13a (in FIG. 2), it is sufficient to use the one-dimensional array sensor if detecting a focus only in a one-directional pattern (longitudinal line or lateral line), and the two-dimensional array sensor is used if it is necessary detect a focus in a two-directional pattern (both longitudinal line and lateral line).
In order to obtain the best focusing surface, the surface of the reference flat mirror 13 is oscillated in the optical axis direction of the exposure lens 9. Corresponding to the position at this point, the apparatus is capable of obtaining information such that the focusing state of the stage reference mark 13a (in FIG. 2) is changed. The apparatus is not specifically limited as to which kind of focusing information is employed as a signal. Focusing information may be employed such that light intensity contrast of an image of the stage reference mark 13a is at the highest level at the best in-focus position and decreased at the defocusing position. Also, a differential value (corresponding to an inclination angle) of a light profile of an image of the stage reference mark 13a may be estimated. These signal processes are performed by an image signal analyzing circuit 47.
The related art has: a first process to focus the wafer surface on the best image-forming surface of the projection lens 9 by first exposing and transferring a pattern on the reticle 6 onto the wafer 12 actually; a second process to detect the best image-forming position on the surface of the wafer 12 or equivalent thereto by the TTL focus detection optical system 27 at a first time almost similar to that of the first process; and a third process to detect the best image-forming position of the surface of the wafer 12 or equivalent thereto by the detection optical system 27 during exposure of the wafer 12 or at a second time different from the first time. Based on the result of detecting the best focus position obtained by the third process, the surfaces of the reticle 6 and wafer 12 are always corrected to the best focusing state.
The next description turns to a specific operation of a conventional TTL auto-focusing method. FIG. 3 is a flowchart showing the sequence of auto-focusing detection. As shown in FIG. 3, first of all, the focus of the detection optical system 27 is coarsely adjusted to the reticle pattern surface 6a (step S1). Originally, the purpose of this step is to prevent a measuring range from being displaced. Next, a focus origin is set between the reticle 6 and the wafer 12 by printing. Specifically speaking, a reticle pattern (for example, a resolution chart or an actual device pattern) is printed at a plurality of positions on the wafer 12, while changing the focus of the wafer surface 12a (steps S2 and S3). The wafer 12 is once developed (step S4). A printed image is measured by means such as an SEM and the best focusing surface is determined (step S5).
On the other hand, at almost the same time as that of the first process (which means that this time is within a sufficiently short time during which the focusing state of the apparatus does not change), the above-discussed detection optical system 27 is used and the focusing measurement is repeated by sending the stage reference mark 13a and changing the focus thereof (steps S6 and S7). The best focusing surface is automatically determined by the use of the signal obtained by the position sensor 26 (step S8).
After the best focusing surface is obtained, this information is inputted to the apparatus as a focus offset (step S9). The offset input means corrects the origin displacement amount between an actual best exposure focus and the best in-focus surface detected by the auto-focusing control system 32. This origin is first set for every process and the information is filed as a process offset.
If the exposure of the actual process wafer starts once, the stage reference mark 13a is periodically sent during exposure of the actual wafer (step S10). The focusing measurement is repeated by the detection optical system 27 while changing the focus, similar to the foregoing (steps S11 and S12). The best focus measuring surface is automatically determined (step S14). The offset change amount between the best focusing surface and the focusing origin obtained at the initial time is calculated (step S14). The wafer stage 11 is driven toward the z-direction by the value of the offset change amount, and the offset amount is corrected so that the wafer surface 12a is moved to the best focusing surface of the projection lens 9 (step S15). Conventionally, the change amount of the focus offset is detected during exposure periodically or at a predetermined timing.
It is advantageous in the TTL auto-focusing system shown in FIG. 1 to provide a projection exposure method and a device manufacturing method, whereby a surface to be exposed can be always matched to an in-focusing surface of the projection optical surface at a high precision even under changing characteristics of the projection optical system and fluctuating of a focus position of the projection optical system due to an environmental change or a repetitive exposure, so that a high-integrated device can be simply manufactured. In other words, the focus of the wafer surface is directly detected through the projection lens, so that it is possible to obtain high in-focusing precision for a focus shift of the projection lens, which is caused by a focusing fluctuation of the projection lens with the elapse of time or exposure. Only by printing a pattern merely at the initial time of setting an origin and confirming the focus is the throughput improved, because of unnecessary printing after that. Thereby, a specific mark for detecting focus is unnecessary on the reticle. As long as there is a glass translucent portion that is within 1 mm on the reticle of the actual process, the apparatus is fully able to obtain a signal from the reference mark or the mark on the wafer through the glass translucent portion. Therefore, this method also can be applied to an existing reticle (having no mark for auto-focusing and no opening window portion), and thus, it is capable of necessarily obtaining a focusing state at a high precision. Further, the reticle pattern is always exposed and transferred onto the wafer in the best focusing state, so that it might be possible to reduce the number of defective semiconductors and largely improve the productivity.
FIG. 9 is a block diagram showing an outline of an off-axis wafer alignment system in, for example, a projection exposure apparatus. It is assumed that in the apparatus, the fine reticle alignment (FRA) executes the alignment for a reticle 91 and a projection exposure optical system 92 precisely, and a positional relation (base line) between a projection exposure optical system 92 and off-axis observation optical systems 94 and 107 have been already measured. A wafer 98 is coarsely positioned by a pre-alignment device 96, thereafter, transferred onto an XY stage 100, and held to a wafer chuck 99 on the XY stage 100 by vacuum suction. A pattern printed on the wafer 98, which has been held on the XY stage 100, is captured and sent to an image storage calculating device 93 as an image signal via a microscope 94 and a CCD camera 107. The captured image is collated with the pattern of the alignment mark, which has been stored in the image storage calculating device 93 by pattern matching. If the alignment mark is detected, an offset amount is calculated. By a series of processes, the system is able to accurately detect the positional offset in the plane direction of the wafer 98.
In order to improve the precision of the wafer alignment process, it is necessary to control a gap between the microscope 94 and the wafer 98 on the XY stage 100, namely, a distance between both of them in the Z-axis direction (vertical direction of the wafer) (labeled xe2x80x9cfocusxe2x80x9d, hereinafter) and to decrease the influence of a focal point blur (referred to as xe2x80x9cdefocusxe2x80x9d, hereinafter) of a pattern as an observing target as much as possible. As for a method of obtaining the best focus by controlling the distance between the microscope 94 and the XY stage 100, there have been hitherto known a method using an auto-focusing mechanism (called an xe2x80x9cimage AFxe2x80x9d, later on) by an image process. The image AF will be now described.
In an alignment system shown in FIG. 9, it is assumed that the XY stage 100 can be driven in the Z-axis direction within a range of Zminxe2x89xa6Zxe2x89xa6Zmax. The image AF utilizes an estimating function to quantify the blur degree of an image by an image process so as to obtain the most proper height of the XY stage, that is, best focus. In other words, an image signal of a pattern on the wafer 98 is captured through the microscope 94 and the CCD camera 107 while the XY stage 100 is shifted toward the height direction. The estimating function at this time is plotted by the image storage calculating device 93, as shown in FIG. 12. A Zbest of the XY stage 100, which is determined to be the highest estimating value, is set to the best focus. In this case, there has been known a method of using a contrast value which is obtained from the image signal of the pattern captured from the CCD camera 107, as an estimating function. It is possible to prevent the defocus of the alignment mark pattern by conducting the aforementioned image AF process as preparation for the alignment process.
However, recently, in accordance with an increasing demand for a fine pattern and a high integration of a semiconductor device, an LSI device, a VLSI device, etc., an image-formation (projection) optical system having a higher resolution has been needed in the projection exposure apparatus. Corresponding to that, a high NA (numerical aperture) of the image-formation optical system has been developed and thus, the focusing depth of the image-formation optical system is made shallow. Mainly, an ASIC, for example, of a multi-kind and a small-lot has been produced, so that improvement of the throughput has been desired in the projection exposure apparatus.
As factors for displacing the best focus position, there are exemplified, a thermal load due to a change in the elapsed time and exposure, a variation in thickness of the wafer, a suction method thereof, wafer deformation, atmospheric change, temperature change, and the like. By contrast, according to the related art shown in FIG. 3, auto-focusing is performed periodically or at a predetermined timing and, therefore, even if the best focus position at the preceding time is almost matched to that at this time, the best focus position is detected by the first process, second process, or third process. Accordingly, the throughput is diminished in proportion to the process for detecting the best focus position. The system is capable of obtaining almost the same measuring value as that of the best focus position when measuring the measuring value near the best focus position. The auto-focusing measurement is conducted, setting the change amount of the relative position of the stage reference mark in the optical axis for the projection optical system. Accordingly, the throughput is diminished by reason of implementing an unnecessary light amount measurement or a contrast measurement at the relative position of the stage reference mark in the optical axis for the projection optical system at the second and subsequent auto-focusing measurement times, so as to obtain the best focus position.
When obtaining the best focusing value by the aforementioned image AF process, using the device shown in FIG. 9, an image must also be captured repetitively, while the XYZ stage is displaced toward the Z-axis direction by a micro amount at one time. In the case wherein the image AF process is effected for every wafer, a drop in the processing speed might be caused if the number of wafers to be processed is increased. There has been known a method whereby the best focusing value, which has been measured by the image AF at the time of processing a first wafer in the same lot, is stored, and with respect to second and subsequent wafers, the best focusing value, which has been stored, is referred to, and the best focusing value is adjusted by driving the XY stage to the corresponding coordinates, thereby omitting the image AF process for second and subsequent wafers, and preventing a decrease in throughput. However, according to that method, the stored best focusing value might often be invalid when processing a lot of the wafers, whose focuses of patterns on the wafers are varied. For example, if a plurality of wafers, which were processed at a pre-process by different apparatuses are mixed in the same lot, the best focusing values of the patterns formed on the wafers might be different for every wafer. In this case, when using the stored best focusing value as it is, this might cause a problem that the alignment mark is set to defocusing and the precision of the alignment process drops.
It is an object of the present invention, in view of the problems in the related art, to provide an exposure apparatus and a device manufacturing method, wherein a focus position is preferably maintained and, simultaneously, the throughput due to the measurement of the best focus position is prevented from decreasing.
To attain the objects, according to an aspect of the present invention, an exposure apparatus comprises a movable stage for holding an original plate, focus measuring means for measuring a best focus position by observing a surface of the original plate, while changing a relative position between the original plate surface and an observation optical system toward an optical axis direction of the observation optical system, and control means for controlling the focus measuring means by determining whether the focus measuring means executes the measurement, in accordance with a relation between a value of a specific measuring parameter and a predetermined value thereof.
According to another aspect of the present invention, an exposure apparatus comprises exposing means for exposing, via a projection optical system, a pattern of an original plate onto a substrate, which is placed on a movable stage, focus measuring means for measuring a best focus position during the exposure, by observing a reference mark on the movable stage or a surface of the substrate, while changing a relative position between the movable stage and the projection optical system toward an optical axis direction of the projection optical system, and control means for controlling the focus measuring means by determining whether the focus measuring means executes the measurement, in accordance with a relation between a value of a specific measuring parameter and a predetermined value thereof.
In one preferred form according to this aspect of the present invention, the measuring parameter denotes an elapsed time after a preceding exposure.
In another preferred form according to this aspect of the present invention, the measuring parameter denotes a light amount or a contrast value of an image being observed by the focus measuring means.
According to further another aspect of the present invention, an exposure apparatus comprises exposing means for exposing, via a projection optical system, a pattern of an original plate onto a substrate, which is placed on a movable stage, focus measuring means for measuring a best focus position during the exposure, by observing a reference mark on the movable stage or a surface of the substrate, while changing a relative position between the movable stage and the projection optical system toward an optical axis direction of the projection optical system, and control means for controlling the focus measuring means by determining whether the focus measuring means executes the measurement, based on an elapsed time after the preceding exposure.
In one preferred form according to this aspect of the present invention, if the elapsed time after the preceding exposure is within a range of a preset time, the control means determines that the focus measuring means does not measure the best focus position, and the best focus position, which the focus measuring means measured at a preceding time, is set to the best focus position upon the exposure.
In another preferred form according to this aspect of the present invention, if the elapsed time after the preceding exposure is greater than a range of a preset time, the control means determines that the focus measuring means measures the best focus position.
According to still a further aspect of the present invention, an exposure apparatus comprises exposing means for exposing, via a projection optical system, a pattern of an original plate onto a substrate, which is placed on a movable stage, focus measuring means for measuring a best focus position during the exposure by observing a reference mark on the movable stage or a surface of the substrate, while changing a relative position between the movable stage and the observation optical system toward an optical axis direction of the observation optical system, thereby obtaining predetermined estimating information, and control means for controlling an operation of the focus measuring means by determining whether the focus measuring means measures a best focus position, on the basis of the predetermined estimating information obtained by the focus measuring means at a predetermined timing and estimating information obtained by the focus measuring means at a preceding time.
In one preferred form according to this aspect of the present invention, the estimating information is a value of a light amount or a contrast value, and the control means performs the determination on the basis of a differential between (i) the value of the light amount or a contrast value, which is obtained by the focus measuring means at a best focus position measured by the focus measuring means at the preceding time and (ii) a maximum value of the light amount or contrast value when the focus measuring means measures the best focus position at the preceding time.
In another preferred form according to this aspect of the present invention, when the differential is within a preset range, the control means determines that the focus measuring means does not measure the best focus position, and the best focus position measured by the focus measuring means at the preceding time is set to the best focus position during the exposure.
In a further preferred form according to this aspect of the present invention, when the differential is greater than a preset range, the control means determines that the focus measuring means measures the best focus position.
In still another preferred form according to this aspect of the present invention, when the focus measuring means measures the best focus position in the case wherein the differential is greater than the preset range, the control means determines change positions of the movable stage in the optical axis direction.
In yet a further preferred form according to this aspect of the present invention, the control means determines, as the change positions, several points in front of and behind the focus position obtained by measuring the best focus position by the focus measuring means at the preceding time, including the best focus position itself.
In a further preferred form according to this aspect of the present invention, the control means causes the focus measuring means to measure the light amount or contrast at several points in front of and behind the best focus position obtained by measuring the best focus position by the focus measuring means at the preceding time, and determines the change positions on the basis of the measurement result.
In still a further preferred form according to this aspect of the present invention, the focus measuring means obtains predetermined estimating information indicative of a defocusing degree of an image being observed, when observing the reference mark on the movable stage or the surface of the substrate.
In yet a further preferred form according to this aspect of the present invention, the focus measuring means sets a position, where the defocusing degree of the image being observed is the lowest, to the best focus position during the exposure.
In a still further preferred form according to this aspect of the present invention, the estimating information is a value of a light amount or a contrast value, and the control means causes the focus measuring means to measure the light amount or contrast at several points in front of and behind the best focus position measured by the focus measuring means at the preceding time.
In yet a still further preferred form according to the present invention, when the focus measuring means detects no best focus position in the case wherein the focus measuring means measures the light amount or contrast, the control means causes the focus measuring means to predict the best focus position from a result of measuring the light amount or contrast by the focus measuring means, and the focus measuring means to measure the best focus position on the basis of the prediction.
According to a further aspect of the present invention, an exposure apparatus comprises a movable stage for holding a substrate, an observation optical system for observing a surface of the substrate, focus measuring means for measuring a best focus position of the observation optical system on the basis of an image signal from the surface of the substrate, while changing a relative position between the movable stage and the observation optical system toward an optical axis direction of the observation optical system, and control means for controlling the focus measuring means by determining whether the focus measuring means measures a best focus position, on the basis of a differential between estimating information obtained by the focus measuring means at a predetermined timing and estimating information obtained by the focus measuring means at a preceding time.
In one preferred form according to this aspect of the present invention, the estimating information is a value of a light amount or a contrast value of an image being observed by the focus measuring means.
In another preferred form according to this aspect of the present invention, the control means performs the determination when the substrate, as a target, is one other than a substrate at a head of a lot.
According to still a further aspect of the present invention, a device manufacturing method comprises an exposing step of exposing, through a projection optical system, a pattern of an original plate onto a substrate placed on a movable stage, a focus measuring step of measuring a best focus position during the exposure by observing a reference mark on the movable stage or a surface of the substrate, while changing a relative position between the movable stage and the projection optical system toward an optical axis direction of the projection optical system, and a controlling step of controlling the focus measuring step by determining whether the focus measuring step executes the measurement, on the basis of an elapsed time after a preceding exposure.
According to yet a further aspect of the present invention, a device manufacturing method comprises an exposing step of projecting and exposing, through a projection optical system, a pattern of an original plate onto a substrate placed on a movable stage, a focus measuring step of measuring a best focus position during the exposure by observing a reference mark on the movable stage or a surface of the substrate and obtaining predetermined estimating information, while changing a relative position between the movable stage and the projection optical system toward an optical axis direction of the projection optical system, a controlling step of controlling the focus measuring step by determining whether the focus measuring step measures a best focus position, on the basis of estimating information obtained at a predetermined timing in the focus measuring step and estimating information obtained at a preceding time in the focus measuring step.
According to a still further aspect of the present invention, a device manufacturing method comprises a focus measuring step of observing a best focus position of an observation optical system on the basis of an image signal from a surface of a substrate while changing a relative position between a movable stage for holding the substrate and the observation optical system toward an optical axis direction of the observation optical system; and
a control step of controlling the focus measuring step by determining whether the focus measuring step measures a best focus position, on the basis of a differential between estimating information obtained at a predetermined timing in the focus measuring step and estimating information obtained upon measuring the best focus position at a preceding time in the focus measuring step.